The right-hand side of Equation (6.22) has an identical form to Equation (6.15),
which we obtained for a standing wave on a taut string. Hence, we have the
important result that a standing wave is the superposition of two travelling waves
of the same frequency and amplitude travelling in opposite directions. This is
illustrated in Figure 6.3, which shows the two travelling waves at successive instants
of time separated by T /8 where T is the period of the wave. The wave travelling
towards the right is represented by the thin continuous curve and the wave travelling
towards the left is represented by the dotted curve. The arrows attached to these
curves indicate the directions of travel. (At some instants of time the two waves lie
on top of each other.) The thick continuous curve is the sum or superposition of the
two travelling waves, i.e. the resultant standing wave. Its overall shape is just like